We compared the carrier status by performing nasopharyngeal swabs for Neisseria meningitidis in 100 pregnant and 99 non-pregnant women.. To test that hypothesis, we planned a cross-secti
Trang 1R E S E A R C H Open Access
A comparison of meningococcal carriage by
pregnancy status
Eric J Knudtson1*, Mike L Lytle2, Beverly A Vavricka1, Valerie S Skaggs3, Jennifer D Peck3, Andrew E Elimian1
Abstract
Neisseria meningitidis is the second leading cause of invasive meningitis A prerequisite for infection is colonization
of the nasopharynx, and asymptomatic carrier rates are widely reported in the range of 10-15% Recent reports have indicated an increased likelihood that a pediatric admission for Neisseria meningitidis will have a mother who
is pregnant in the home We hypothesized that this association may relate to immunologic changes in pregnancy leading to higher carrier rates
We compared the carrier status by performing nasopharyngeal swabs for Neisseria meningitidis in 100 pregnant and 99 non-pregnant women
Average age of the participants was 28.9 +/- 6.7 years The average gestational age at specimen collection was 27.5 +/- 9.4 weeks Non pregnant women were significantly more likely to use tobacco (38% vs 24%, p < 0.0001)
In the entire 199 patients, only one pregnant patient tested positive for Neisseria meningitidis (0.5%; 95% CI: 0.01%-2.8%)
The meningococcal carrier rate in our population is well below what is widely reported in the literature Assuming
a 1% carrier rate in the pregnant group and a 0.5% carrier rate in the non pregnant group, 4,763 patients would
be required to detect a difference of this magnitude, given 80% power and an alpha of 0.05
Background
Neisseria meningitidis, simply known as meningococcus,
is a gram negative bacterium and a leading cause of
bac-terial meningitis Nasopharyngeal carriage is a
prerequi-site for invasive disease, and approximately 10-15% of
healthy individuals are reported to carry the organism at
any one time Rates of carriage and transmission are
known to increase in closed, or semi-closed living
con-ditions such as military barracks, jails, and college
dor-mitories [1,2] Additional factors shown to affect the
carriage rate are: age, gender, social class, exposure to
cigarette smoke, and vaccination [3,4]
Recent information demonstrates that pediatric disease
may correlate with a pregnant mother For example, van
Gils et al evaluated 176 hospitalized children, half of
whom were admitted for invasive meningococcus
Amongst the cases 19% of children were found to have
a mother who was pregnant, compared to only 2% of
controls Multivariate analysis showed meningococcal disease was 11.7 times more likely to occur in a child whose mother was pregnant [5]
One mechanism may be the immunologic changes of pregnancy predispose a woman to being an asympto-matic carrier To test that hypothesis, we planned a cross-sectional observational study of nasopharyngeal carriage of meningococcus to compare rates by preg-nancy status
Methods
This was an observational study Institutional Review Board approval was sought and obtained Women pre-senting for care at the University of Oklahoma Health Sciences Center obstetrics and gynecology clinics were invited to participate Samples were obtained from Sep-tember 23, 2008 until May 21, 2009 Eligible women included pregnant and non-pregnant women aged 18-45 years Women were excluded if they had twins or higher order multiple gestations Non-pregnant women were excluded if they had a pregnancy in the preceding nine months Additional exclusion criteria included known or
* Correspondence: eknudtso@ouhsc.edu
1 Department of Obstetrics and Gynecology, Division of Maternal Fetal
Medicine, The University of Oklahoma Health Sciences Center, PO Box 26901,
WP 2470, Oklahoma City, OK, 73160 USA
Full list of author information is available at the end of the article
© 2010 Knudtson et al; licensee BioMed Central Ltd This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
Trang 2suspected meningococcal vaccination or antibiotic use in
the preceding month
Women who agreed to participate had their
demo-graphic information recorded and a nasopharyngeal
swab performed by a research nurse Recorded
demo-graphic information included age, race, gravidity, parity,
estimated gestational age at enrollment, estimation of
annual household income, exposure to cigarette smoke,
number and ages of members residing in household,
and occupation
The nasal swab was performed in a standardized
man-ner Briefly, after explaining the procedure, an Aimes
culturette swab with charcoal® was passed through the
nares until resistance was met The nasopharynx was
sampled for 30 seconds and the swab was removed
Spe-cimens were labeled with a study number and the
legend was kept in a separate secure location
The specimens were then immediately transported to
the Oklahoma State Department of Health where they
were immediately plated on Modified Thayer Martin
agar Plates were incubated at 35-37 degrees C in 5-8%
CO2 at 18-24 hours and 36-48 hours Those specimens
identified as suspect for Nisseria meningitidis were
tested for gram stain and oxidase testing Specimens
that were positive for gram-negative diplococci and
oxi-dase test positive were plated on chocolate agar to
incu-bate for 18-24 hours After incubation, identification of
the isolate was performed using the Biomerieux Vitek
API NH system The generated code number was
entered into the API software, with a result of Neisseria
meningiditis with a greater than 85% probabilities was
considered definitive identification Each isolate
identi-fied as Neisseria meningitidis was plated on Heart
infu-sion agar with 5% sheep blood and incubated for 18-24
hours After incubation, specimens were then serotyped
Ournull hypothesis was that pregnant women were
not more likely to be carriers of Neisseria meningitidis
The difference in proportion of carriers by pregnancy
status was evaluated using a Fisher’s exact chi-square
test Pearson or Fisher’s Exact chi-square tests were
per-formed to determine if the distribution of demographic
characteristics differed by pregnancy status Differences
in mean age were examined using a Student’s t-test In
a cross sectional study designed to evaluate college
dor-mitory students in the United Kingdom, students
begin-ning the fall term in October had a baseline carriage
rate of 14% One month later the carriage rate was 31%,
(a 120% increase) [6]
Assuming a similar baseline carrier rate, a sample size
of 200 (100 pregnant women and 100 non-pregnant
women) provided for 80% power to detect a 16% to 18%
absolute difference (a 120% relative increase) in carriage
rates between pregnant and non-pregnant women using
an alpha of 0.05
Results
Two-hundred and sixty one patients were evaluated and fifty five either declined to participate or were ineligible due to age or recent antibiotic use Nasopharyngeal swabs were performed on 206 women (103 pregnant and 103 non-pregnant) The six additional cultures beyond the sample size calculation were performed after
a review showed that 4 patients were swabbed despite meningococcal vaccination, and 3 samples never had final results reported by the Oklahoma State Depart-ment of Health The final cohort consisted of 199 patients, (100 pregnant and 99 non-pregnant) Table 1 Table 1 Characteristics of the study group
Pregnant (n = 100)
N or Mean (sd)
Nonpregnant (n = 99)
N or Mean (sd)
p
Age (mean, sd) 27.60 (5.90) 30.17 (7.22) NS a
Parity
Race Caucasian 39 60 0.0002 c
African American 12 20 Hispanic 40 15 Native American 5 2
Estimated Gestational Age
27.49 (9.36) N/A N/A Tobacco Use 14 38 <0.0001 c
Number in Household
1 2 16 0.0106 c
>5 19 15 Median Income
<20,000 23 17 0.5951c
20,000-39,999
20 19
40,000-59,999
7 13
>60,000 10 11 Unknown 39 40
a
Student ’s t-test
b Fisher’s exact chi square test
c Pearson’s Chi-square test
Trang 3shows the demographic characteristics and descriptive
statistics for the study population The average age of all
participants was 28 9 standard deviation (sd 6.7 years)
Gestational age ranged from 7 to 40 weeks, with a mean
of 27.5 weeks (sd 9.4) There was a significant difference
between the pregnant and non-pregnant women in
terms of race and smoking status In the entire group of
199 patients, only one carrier was identified (0.5%; 95%
CI: 0.01%-2.8%) She was a 23 year old African
Ameri-can woman She was pregnant and in her 39thweek of
pregnancy She smoked, was unemployed and was found
to have meningococcal serotype group X
Discussion
Disease from Neisseria meningitidis remains a
signifi-cant public health problem Nasopharyngeal carriage is
a prerequisite for invasive infection and the proportion
of asymptomatic carriers varies based on a number of
factors, with higher rates associated with increased
dis-ease incidence [2,6,7] Recently, van Gils published work
describing pediatric hospital admissions for invasive
Neisseria meningitidis Patients with meningitis were
nearly 12 times as likely to have a mother who was
pregnant in the household as compared to controls [5]
If true, how might pregnancy confer an increased risk
to another member in the household? One plausible
explanation is that pregnancy confers a transient
increase in the maternal carriage of Neisseria
meningiti-dis Susceptible household members such as young
chil-dren would then be more prone to transmission and
active disease To test this hypothesis we designed a
simple observational study to compare the proportion of
carriers in our pregnant and non-pregnant populations
Our sample size was based on repeated estimates of the
general population carriage rate being approximately
10-15% [1]
To our surprise, the entire cohort of 199 patients
showed only one positive result There are several
possi-ble explanations The first is that there were specimen
collection and/or laboratory errors that led to a
systema-tic error in the classification of carrier status Our
review shows this to be an unlikely explanation for
these results At the halfway point of patient
recruit-ment, we reviewed our specimen collection, transport,
and processing procedures and found them to be
consis-tent with published reports [1,4,8] Also, our specimens
were analyzed at the Oklahoma State Department of
Health, which has an experienced microbiology lab A
review of the laboratory techniques also showed no
lapse in quality control Laboratory procedures were
consistent with published standards [1,4,8] (Michael
Lytle, OSDH, personal communication)
The more likely explanation appears to be that
menin-gococcal carriage has decreased significantly over time
This can be reasonably inferred by looking at the disease incidence in the state of Oklahoma Over a twelve year time span the meningococcal disease incidence rate decreased from approximately 1.5 to 0.5/100,000 [9] Additionally, a requested review of results at the microbiology lab at the University of Oklahoma showed that in the 15 months prior to completing this study, there was not one report of an incidental finding of Neisseria meningitidis from any nasopharyngeal cultures taken for routine clinical care [personal communication, Mary Magnus, director, OUHSC microbiology lab] The underlying mechanism for the lower than expected carriage rate cannot be explained by this study Carriage, transmission and disease burden vary naturally over time Additionally, use of the MC4 vaccine has expanded In particular, the change in the recommenda-tions of the Advisory Committee on Immunization Prac-tices, which expanded vaccination to all adolescents, may influence the carriage rate in the general popula-tion While vaccination was an exclusion criterion for participation in this study, a herd immunity effect is possible
Finally, our decision to use nasopharyngeal cultures may also have had an impact on our carrier rate In some reports, the nasopharyngeal culture resulted in a slightly lower sensitivity when compared to the orophar-yngeal technique [10]
Conclusions
In summary, there was no significant difference in car-riage of Neisseria meningococcus in relation to preg-nancy status Only one carrier was identified in the entire study population of 199 patients Thus, the 0.5% (95% CI: 0.01%-2.8%) carriage rate observed among women at the University of Oklahoma obstetrics and gynecology clinics is well below estimates reported for other populations
Acknowledgements
We thank Shelly Hopper for assistance with patient recruitment Author details
1 Department of Obstetrics and Gynecology, Division of Maternal Fetal Medicine, The University of Oklahoma Health Sciences Center, PO Box 26901,
WP 2470, Oklahoma City, OK, 73160 USA 2 Oklahoma State Department of Health, 1000 NE 10th, Oklahoma City, OK 73117 USA.3The University of Oklahoma Health Sciences Center College of Public Health, 801 N.E 13th St Oklahoma City, OK 73104 USA.
Authors ’ contributions EJK conceived the study, designed the experiment, helped with data acquisition and analysis and wrote the manuscript MLL performed the microbiologic cultures BAV was primarily responsible for patient recruitment and culture sampling VSS assisted with study design, and performed data analysis JDP assisted with study design, data analysis and manuscript preparation AEE assisted with manuscript preparation All authors wrote and approved the final manuscript.
Trang 4Competing interests
The authors declare that they have no competing interests.
Received: 5 May 2010 Accepted: 11 August 2010
Published: 11 August 2010
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doi:10.1186/1477-5751-9-6
Cite this article as: Knudtson et al.: A comparison of meningococcal
carriage by pregnancy status Journal of Negative Results in BioMedicine
2010 9:6.
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